This book examines the rapid development of the fundamental concept of a crime in international criminal law from a comparative law perspective in light of the catalyzing impact of the criminal law theory developed in major world legal systems on the crystallization of the substantive part of international criminal law. This study offers a critical overview of international and domestic jurisprudence in regards to the construal of the concept of a crime (actus reus, mens rea, defences, modes of liability) and exposes roots of confusion in international criminal law through a comprehensive comparative analysis of substantive criminal laws in selected legal jurisdictions.